Seal it up and put it in the fridge, and it will last for years.
However, it will darken over time, and even the lightest LME will eventually look like Dark LME. Some people think it gets less fermentable with time, but I suspect for most people that is from hearsay, and they have not actually experienced it.
My 33 lb containers last a few months at basement temps (60 - 70 F), with no issues. They are kept in the dark, and I don't notice any discoloration over that time.
By all means try and us it within 'a couple of weeks', but here is no reason to toss it out if not used in a few months.